The present invention relates to an electronic starter circuit for a fluorescent lamp.
Fluorescent light control differs from many electronics applications in that very high reliability is necessary, and low cost is also a very important objective.
The electrical behavior of the fluorescent lamps that contain pressurized gases (such as neon or argon) is similar to that of an avalanche zener diode with a resistance in the gas that becomes very low and negative after the breakdown.
A starter is needed to obtain the breakdown of the gas in the lamp: what has to be done is to prompt a surge voltage at the terminals of the lamp to ionize the gas. Thus it is usual to use an inductor and a device to short circuit the lamp and make the current flow. When the device is opened, the energy stored in the inductor gets converted into a surge voltage that causes the breakdown of the gas.
It is also preferable to preheat the filaments of the lamp in order to carry them to a temperature at which they easily emit electrons.
In practice, the duration of the short circuit is standardized depending on the category of lamp considered. For example, it is about 1.5 seconds for low-pressure fluorescent lamps. Standardization means that a lamp made by a given manufacturer can be used with a starter made by another manufacturer.
One type of commonly used starter is a two-strip starter (with a parallel-connected anti-parasitic capacitor). This low-cost electromechanical device enables a short circuit to be held for the fairly lengthy preheating time needed. (1.5 seconds), and then the opening of the short circuit to break down the gas.
However, the lamp does not always come on. So long as it is not lit (namely so long as the breakdown has not taken place), the two-strip device will continue to cycle. There is then a permanent flicker of the lamp that is particularly bothersome. Furthermore, the two-strip device may be damaged. Finally, the two-strip device opens for any current whatsoever. It may therefore open when the current is almost zero: the energy is then far too small to be efficient. It may also open at a time when the current is excessively high: then the lamp itself may be damaged.
To overcome these drawbacks, it has been sought to use electronic circuits, using in general a triac or a thyristor as a power device and a counter to limit the number of attempts made in order to light up the lamp.
These proposed circuits consume a great deal of current. They require very high capacitance for the capacitor used to hold the logic voltage needed to control the electronic power device (triac) and for the counting throughout the duration of the short circuit.
Furthermore, these circuits must measure the duration of the short circuit in order to then activate the opening of the electronic power device. Now this preheating time is relatively lengthy. It has been seen, in one example, that it had a standardized value of 1.5 seconds (for a low-pressure fluorescent lamp). It is not desirable to use a slow RC circuit to measure a period of such length. In particular, this is because it becomes necessary, in this case, to use a high resistance (of 1 megohm, for example) that reduces immunity to noise (with high input impedance).
It is preferred to use a fairly fast RC circuit followed by a high-capacity counter to measure the desired duration.
However, these electronic circuits consume a great deal of current and, in practice, they require an electrochemical type of high-capacitance capacitor to maintain the logic voltage, i.e. a capacitor with a capacitance of 100 or 1000 microfarads for example. This furthermore makes the circuits more fragile owing to the limited lifetime of these capacitors.
The present application discloses inventions which are intended to overcome these different drawbacks. The inventions relate to an electronic starter of a fluorescent lamp comprising a power switch parallel-connected with the lamp and supplied at high voltage, a gate-control circuit of said switch comprising a circuit for the measurement of a determined preheating time, and a auxiliary supply circuit parallel-connected with said switch and comprising a capacitor to give a logic supply voltage to the gate-control circuit on a terminal of said capacitor.
According to the invention, the preheating time measurement circuit comprises a comparator with two voltage references, a first voltage reference greater than a second voltage reference, the comparator having one input connected to the terminal of the capacitor and one output connected to a logic circuit to switch the first voltage reference over to the comparator when the high voltage is turned on, and so that:
upon the detection of an input voltage corresponding to the first voltage reference, it will deliver a start-of-preheating detection signal to activate the closing of the power switch and switch the second voltage reference over to the comparator, PA0 upon the detection of an input voltage corresponding to the second voltage reference, it will deliver an end-of-preheating detection signal to activate the opening of the power switch.
Advantageously, the gate-control circuit furthermore comprises a preheating current measurement circuit to deliver a signal to activate the opening of the switch after reception of the end-of-preheating detection signal, upon the detection of an optimum preheating current in the lamp.
According to another characteristic of the invention, to reduce the consumption current during the measurement of the preheating time, the gate-control circuit comprises a gate circuit that is placed between the logic supply voltage and the current measurement circuit and controlled by the logic circuit to turn the current measurement circuit off upon detection of the first voltage reference and turn it on upon detection of the second voltage reference.